1. Field of the Invention
The present invention relates to etchable copper conductive foils useful for preparing printed circuit boards and particularly to electrodeposition procedures and electrolyte bath solutions for controlling the properties of such foils. More specifically the invention relates to procedures and electrolyte bath solutions useful for controlling foil properties such as roughness, elongation, tensile strength and ductility. By controlling these properties the electrodeposition operation may be conducted with greater efficiency.
2. Description of the Prior Art
Printed circuit board (PCB) components have become widely used in a variety of applications for radios, televisions, computers, etc. Of particular interest are multi-layer PCB laminates which have been developed to meet the demand for miniaturization of electronic components and the need for PCBs having a high density of electrical interconnections and circuitry. In the manufacture of PCBs, raw materials, including conductive foils, which are usually copper foils, and dielectric supports comprising organic resins and suitable reinforcements, are packaged together and processed under temperature and pressure conditions to produce products known as laminates. The laminates are then used in the manufacture of PCBs. In this endeavor the laminates are processed by etching away portions of the conductive foil from the laminate surface to leave a distinct pattern of conductive lines and formed elements on the surface of the etched laminate. Laminates and/or laminate materials may then be packaged together with etched products to form multi-layer circuit board packages. Additional processing, such as hole drilling and component attaching, will eventually complete the PCB product.
With multiple-layer circuit boards, it may be appreciated that variations in the thickness of the copper foil will lead to nonuniformities of transmission line characteristics within the foil and result in unpredictable electrical characteristics for any given PCB. As the integrated circuits increase in speed, the problem becomes more serious. The dielectric constant and thickness of the substrate along with the height, width, spacing and length of the conductive traces determine many of the electrical performance characteristics of the PCB.
The PCB industry's push toward miniaturization and increased performance per package is resulting in conductors of ever smaller widths, more closely spaced on thinner substrates. The increase in switching frequency of solid state electronic devices that are interconnected by the copper traces, results in further demands on the PCB due to the "skin effect" which results during high frequency operation along a conductor. The characteristics of the copper foil have a significant effect on the electrical performance of the finished PCB. The metallurgical properties of the copper foil are also important in the PCB production process. For example, a foil used in multi-layer laminates must not crack during hole drilling. Also, foils which are less susceptible to wrinkling during the lamination process are preferable for reducing scrap losses.
The production of copper foil by electrodeposition processes involves the use of an electroforming cell (EFC) consisting of an anode and a cathode, an electrolyte bath solution, generally containing copper sulphate and sulphuric acid, and a source of current at a suitable potential. When voltage is applied between the anode and cathode, copper deposits on the cathode surface.
The process begins by forming the electrolyte solution, generally by dissolving (or digesting) a metallic copper feed stock in sulphuric acid. After the copper is dissolved the solution is subjected to an intensive purification process to ensure that the electrodeposited foil contains no disruptions and/or discontinuities. Various agents for controlling the properties may be added to the solution.
The solution is pumped into the EFC and when voltage is applied between the anode and cathode, electrodeposition of copper occurs at the cathode. Typically, the process involves the use of rotatable cylindrical cathodes (drums) that may be of various diameters and widths. The electrodeposited foil is then removed from the cylindrical cathode as a continuous web as the cathode rotates. The anodes typically are configured to conform to the shape of the cathode so that the separation or gap therebetween is constant. This is desirable in order to produce a foil having a consistent thickness across the web. Copper foils prepared using such conventional electrodeposition methodology have a smooth shiny (drum) side and a rough or matte (copper deposit growth front) side. Conventionally such foils are bonded to dielectric substrates to provide dimensional and structural stability thereto, and in this regard, it is conventional to bond the matte side of the electrodeposited foil to the substrate so that the shiny side of the foil faces outwardly from the laminate. In a commercial sense, useful dielectric substrates may be prepared by impregnating woven glass reinforcement materials with partially cured resins, usually epoxy resins. Such dielectric substrates are commonly referred to as prepregs.
In preparing laminates, it is conventional for both the prepreg material and the electrodeposited copper foil material to be provided in the form of long webs of material rolled up in rolls. The rolled materials are drawn off the rolls and cut into rectangular sheets. The rectangular sheets are then laid-up or assembled in stacks of assemblages. Each assemblage may comprise a prepreg sheet with a sheet of foil on either side thereof, and in each instance, the matte side of the copper foil sheet is positioned adjacent the prepreg so that the shiny sides of the sheets of foil face outwardly on each side of the assemblage.
The assemblage may be subjected to conventional laminating temperatures and pressures between the plates of laminating presses to prepare laminates comprising sandwiches of a sheet of prepreg between sheets of copper foil.
The prepregs used conventionally in the art may consist of a woven glass reinforcement fabric impregnated with a partially cured two-stage resin. By application of heat and pressure, the matte side of the copper foil is pressed tightly against the prepreg and the temperature to which the assemblage is subjected activates the resin to cause curing, that is cross linking of the resin and thus tight bonding of the foil to the prepreg dielectric substrate. Generally speaking, the laminating operation will involve pressures in the range of from about 250 to about 750 psi, temperature in the range of from about 350.degree. to 450.degree. F. and a laminating cycle of from about 40 minutes to about 2 hours. The finished laminate may then be utilized to prepare PCBs.
Conductive foils for PCB applications are conventionally treated, at least on the matte side, for enhanced bonding and peel strength between the matte side and the laminate Typically the foil treatment involves treatment with a bonding material to increase surface area and thus enhance bonding and increase peel strength. The foil may also be treated to provide a thermal barrier, which may be brass, to prevent peel strength from decreasing with temperature. Finally, the foil may be treated with a stabilizer to prevent oxidation of the foil. These treatments are well known and further description thereof is not necessary at this point.
A number of manufacturing methods are available for preparing PCBs from laminates. Additionally, there is a myriad of possible end use applications. These methods and end uses are known and need not be discussed in detail here. But suffice it to say that each method and each end use has its own set of idiosyncracies which often may dictate the physical and/or chemical characteristics of the foil itself. Thus, the industry has established a set of definitions and has defined eight separate categories or classes of copper foil. These definitions and the characteristics of each of the eight classes of foil are set out in a publication of the Institute for Interconnecting and Packaging Electronic Circuits (IPC) entitled "Copper Foil for Printed Wiring Applications" and designated IPC-CF-150X.
The document IPC-CF-150X, where X denotes, in alphabetical order, the various revisions, contains the specifications for the acceptable technical properties and performance of copper foils for the manufacture of printed circuits. This document in its entirety is contained in military specification MIL-P-13949 for polymeric dielectric laminates and bonding sheets to be used in the production of printed circuit boards. Therefore, certification of foils to the IPC-CF-150X standards automatically ensures qualification to military specification standards.
The current IPC publication is Revision E published May, 1981 and designated IPC-CF-150E and this publication is hereby specifically incorporated herein by reference.
IPC-CF-150E sets forth the following table which defines the minimum values for certain mechanical properties for each class of foil
TABLE 1 __________________________________________________________________________ Mechanical Properties (Minimum Values) (The copper foil shall conform to the tensile and elongation requirements when tested in both longitudinal and transverse directions.) AT ELEVATED AT ROOM TEMPERATURE 23.degree. C. TEMPERATURE 180.degree. C. Copper % DUCTILITY % Elongation Type Copper** Tensile Strength Elong. (2.0" G.L.) Tensile Strength (2.0" G.L.) and Weight Mega Pascals CHS Fatigue Mega Pascals CHS Class oz. Lbs./In..sup.2 (MPa) 2"/Minute Ductility Lbs./In..sup.2 (MPa) 0.05"/Minute __________________________________________________________________________ TYPE 1 1/2 15,000 103.35 2.0 Not developed NOT APPLICABLE E 1 30,000 206.70 3.0 2+ 30,000 206.70 3.0 2 1/2 15,000 103.35 5.0 Not developed NOT APPLICABLE 1 30,000 206.70 10.0 2+ 30,000 206.70 15.0 3 1/2 15,000 103.35 2.0 Not developed -- -- -- 1 30,000 206.70 3.0 20,000 137.80 2.0 2+ 30,000 206.70 3.0 25,000 172.25 3.0 4 -- -- -- -- Not developed -- -- -- 1 20,000 137.80 10.0 15,000 103.35 4.0 2+ 20,000 137.80 15.0 15,000 103.35 8.0 TYPE 5 1/2 50,000 344.50 0.5 30.0 -- -- -- W 1 50,000 344.50 0.5 20,000 137.80 2.0 2+ 50,000 344.50 1.0 40,000 375.60 3.0 6 1 25,000 to 172.25 to 1.0 to 30.0 to NOT APPLICABLE 50,000* 344.50 20.0 65.0 2+ according according to according to according to to temper temper temper temper 7 1/2 15,000 103.35 5.0 65.0 -- -- -- 1 20,000 137.80 10.0 14,000 96.46 6.0 2+ 25,000 172.25 20.0 22,000 151.58 11.0 * 1/2 15,000 103.35 5.0 25.0 NOT APPLICABLE 8 1 20,000 137.80 10.0 __________________________________________________________________________ *Properties given are following a time/temperature exposure of 15 minutes at 177.degree. C. (350.degree. F.). **Minimum properties for testing copper weights less than 1/2 ounce shall be agreed to between user and vendor.
Of the classes of foil defined by IPC-CF-150E, perhaps the most widely used by the industry is IPC Class 1 which is sometimes referred to simply as standard foil. IPC Class 1 foil is an electrodeposited foil that has a generally acceptable room temperature ductility characteristics Another class of foil that is widely used by the industry is IPC Class 3 foil which is also sometimes referred to as high elevated temperature ductility foil. IPC Class 3 foil is an electrodeposited foil that has high ductility at elevated temperatures and thus is able to withstand the stresses and strains imposed particularly at thru holes by differential thermal expansion during soldering operations and in high temperature use applications.
Copper foils have been produced for PCB use by two major methods, rolling or electrodeposition. The invention of the present application relates to the latter. As set forth above, to produce copper foil by electrodeposition, a voltage is imposed between an anode and a cathode immersed in a copper containing electrolytic bath solution. Copper is electrodeposited on the cathode in the form of a thin metal film. The qualities and characteristics of the metal film are a function of many parameters such as current density, temperature, substrate material, solution agitation and electrolyte solution composition. Additives are often placed in the electrolyte solution so that the electrodeposit may be formed with certain desired qualities, a main one of which is a certain controlled roughness. Without the presence of additives the copper deposits have the tendency, as a result of crystalline imperfections and grain boundaries, to grow with an uncontrolled roughness that is not uniform. Additionally, a certain controlled degree of roughness is often desired by the copper foil user so that the bond strength is increased between the copper foil and the dielectric support to which the copper foil is adhered, and the copper foil. Roughness contributes to the strength of the bond by increasing the surface area available for bonding.
A gelatine component may often be included in the electrolyte solution to control roughness. The gelatine component most commonly used is animal glue. Glue is believed to function by adsorbing onto the electroplating surface to thus decrease the exchange current density for copper deposition, a condition found in the theories of electrodeposition to be favorable to production of smoother deposits.
According to known methodology, copper foil may be electrodeposited from an electrolyte solution containing about 100 grams per liter (g/l) of copper, about 80 g/l of sulphuric acid and about 80 parts per million (ppm) of chloride ions. Glue may be added to the solution during electrodeposition at addition rates ranging from about 1/2 milligram of glue per minute per 1,000 amperes (mg/min.kA) up to about 11 mg/min kA. The process generally was conducted at a temperature of about 60 degrees centigrade (.degree.C.) using a current density between about 200 and about 1,400 amperes per square foot (ASF). It has been determined that the roughness of the matte surface of the deposited copper foil generally increases as the glue addition rate is decreased and/or the current density is increased. Electrolyte flow is maintained so that plating occurs below the mass transfer limited current density. In such process the glue addition rate may be varied to vary the metallurgical properties of the copper foil to meet various performance criteria. Typical matte side roughnesses (R.sub.tm) of copper foils produced by this method, as measured on a Surftronic 3 profilometer (Rank Taylor Hobson Ltd.--Leicester, England), range from about 4.75 .mu.m to about 8 .mu.m for 1/2 oz. copper foil; from about 6.5 .mu.m to about 10 .mu.m for 1 oz copper foil; and from about 8.75 .mu.m to about 15 .mu.m for 2 oz. copper foil. IPC Class 1 foils were produced by this method at glue addition rates between 5 and 11 mg/min kA while IPC Class 3 foils were produced at glue addition rates less than 5 mg/min.kA. Low profile (low roughness) foils could be produced by increasing the glue addition rate to above 11 mg/min kA.
One disadvantage of the known method is that as the copper foil is deposited to greater thicknesses, the roughness increases and the number of isolated roughness elements also increases. The isolated roughness elements may be separated by interspersed smooth areas, rendering the copper foil unusable for some critical electronic applications. To minimize such roughness increases, the current must be decreased, resulting in lost production capacity. Another disadvantage of the known methodology is that copper foils with lower matte side roughnesses (low profile foils) were not readily obtainable without corresponding decreases in metallurgical qualities, such as in elevated temperature elongation Thus, low profile IPC Class 3 foils generally could not be produced by the known methods without substantial loss of efficiency. In this regard, the lowest R.sub.tm achievable using known methods for producing IPC Class 3 foils was approximately 11 to 12 .mu.m for 2 oz. foil, approximately 7 to 8 .mu.m for 1 oz. foil, and approximately 5 to 6 .mu.m for 1/2 oz. foil. Furthermore, the lowest R.sub.tm achievable using the known methods for producing Class 1 foils was approximately 5.2 .mu.m for 2 oz. foil, about 5 .mu.m for 1 oz. foil, and about 4.6 .mu.m for 1/2 oz. foil. On the other hand, low profile foils having a R.sub.tm below about 4.5 .mu.m have sometimes become desirable because they provide finer line definition, better impedance control and reduced propagation delays. In particular low profile foils are desirably used to facilitate tape automated bonding operations. In general, low roughness facilitates the use of less resin in bonding the foil to a dielectric substrate as well as the use of thinner laminates.
The prior art processes described above and which have utilized glue in an attempt to control properties such as roughness and ductility, have suffered from several distinct and difficult disadvantages. Firstly, when the process was used to make standard IPC Class 1 foils, the overall efficiency of the process was limited by the fact that increases in current density (ASF) were generally accompanied by increases in roughness and decreases in ductility. Secondly, to decrease roughness and produce low profile foils it was necessary to increase the glue addition rate; however, an increased glue addition rate resulted in decreased ductility. So it was necessary to reduce the current density to counteract the loss of ductility. Finally, to produce a suitable IPC Class 3 foil it was necessary to decrease the glue addition rate, but this caused an increase in roughness. So in this case it was also necessary to reduce the current density to counteract the increased roughness.
It was generally known in the copper electrorefining industry that surface active agents could be used to produce bulk copper cathodes having smoother surfaces. The smoother surfaces are desirable in the refining industry from the standpoint of plant efficiency. The copper cathodes in a refining plant are deposited to substantial thicknesses, i.e., many millimeters. As the deposit grows to such thicknesses, the cathode becomes rougher, and in extreme cases, dendrites and nodules form on the cathodes causing shorts in the cells. When such shorting occurs plating ceases. To maintain the plating output, the cathode must be changed before a short occurs. In order to minimize the number of cathode changes and to deposit a greater thickness of copper for each cathode, the electrorefining industry has used addition agents such as animal glue, chloride ion and thiourea to reduce dendrite and nodule formation on the copper cathodes.
The electrorefining industry has sought to prepare higher quality copper cathodes, for instance, for use as raw material in super-fine copper wire. E. H. Chia et al., "Organic Additives: A Source of Hydrogen in Copper Cathodes," Journal of Metals, April, 1987, pages 42-45. Chia et al. discuss the use of organic additives including combinations of thiourea and glue, in electrorefining copper. Chia et al. specifically address the assumption that thiourea contributes hydrogen at the cathode in the refining tank.
S. E. Afifi et al., "Additive Behavior in Copper Electrorefining," Journal of Metals, February, 1987, pages 38-41, also discuss the use of organic additives such as gelatine and thiourea in electrodeposition processes. This paper states that such additives can be used to modify the crystal size of the deposit to improve the brightness of the deposit. The authors conclude that small concentrations of thiourea act beneficially toward surface brightness of deposits up to very high values of current densities, but at higher concentrations of thiourea the surface brightness of deposits decreases rapidly, possibly due to increased precipitation of cupric sulfide or sulphur on the electrode surfaces.
Knuutila et al. "The Effect of Organic Additives on the Electrocrystallization of Copper," The Electrorefining of Copper, examine the behavior of thiourea, animal glue and chloride ions in the electrolysis of copper. This paper deals with changes in polarization curves in the electrolysis of copper and indicates that the microstructure of a copper cathode obtained from an electrolyte containing thiourea differs drastically from the microstructure obtained from a bath containing only glue. Specifically, the thiourea field-oriented structure is evident, and the grain size is much finer. Furthermore, animal glue provides a microstructure in deposited copper having a basis-oriented structure.
Ibl et al., "Note on the Electrodeposits Obtained at the Limiting Current," Electrochimica Acta, 1972, Vol. 17, pages 733-739, disclose the use of thiourea in acid cupric sulfate solutions as a leveling agent.
Franklin, "Some Mechanisms of Action of Additives in Electrodeposition Processes," Surface and Coatings Technology, Vol. 30, pages 415-428, 1987, discusses the use of a number of additives in electrodeposition processes, including one for copper.
While the surface deposit effects of glue and thiourea are thus well known to those skilled in the art of copper electrorefining to produce thick copper deposits, the use of thiourea has not previously been applied in the copper foil industry for controlling process parameters or properties such as surface roughness, tensile strength, elongation and/or ductility of thin copper foils to be used in the PCB industry.